[LU-33] Use SDK as AAR from Maven Central

* Depends on core-sdk's [LU-32] (for documentation update)
* Remove LevelUp SDK submodule reference
For SDK development: Use SDK as git submodule

This reverts commit 768f72aea19cab76cf965ca313ed8e14bfc184ee.
Updates submodule to SDK v2.1.2 to match AAR reference on master.
This branch is meant for ease of internal SDK development use; most
developers should use the AAR Maven reference to the SDK instead.